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Abstract:

A method for cleaning a substrate having organic and inorganic residues
disposed thereon is provided. The method includes removing organic
residue from the substrate using atmospheric oxygen plasma, and removing
inorganic residue from the substrate using cryogenic CO2. The
substrate may be pretreated using a benign cooling agent, and
post-treated using a dilute wet chemical cleaning method.

Claims:

1. A method for cleaning a substrate having organic and inorganic
residues disposed thereon, comprising: removing organic residue from the
substrate using atmospheric oxygen plasma; and removing inorganic residue
from the substrate using cryogenic CO.sub.2.

2. The method according to claim 1, further comprising pre-treating the
substrate using a benign cooling agent.

3. The method according to claim 2, wherein the benign cooling agent is
liquid N.sub.2.

4. The method according to claim 2, wherein the pretreating includes
submerging the substrate in the cooling agent.

5. The method according to claim 1, further comprising post-treating the
substrate using dilute wet chemistry cleaning.

6. The method according to claim 1, wherein the atmospheric oxygen plasma
is produced by a jet apparatus that includes concentric, inner and outer
electrodes through which a mixture of helium and other gases flow in the
presence of a voltage field.

7. The method according to claim 6, wherein radio frequency energy is
applied to the inner electrode to generate a voltage between 100 V and
250 V.

8. The method according to claim 6, wherein the gas velocity exiting the
jet apparatus is about 10 m/s and the effluent temperature is about
150.degree. C.

9. The method according to claim 6, wherein the jet apparatus generates
an atomic oxygen flux of about 1.times.10.sup.18 atoms/cm2-s.

10. The method according to claim 1, further comprising removing loosened
organic residue from the substrate using cryogenic CO.sub.2.

11. A method for cleaning a substrate having organic and inorganic
residues disposed thereon, comprising: pre-treating the substrate using
liquid N2 provided as a shower or a spray; removing organic residue
from the substrate using an atmospheric oxygen plasma jet; removing
organic and inorganic residue from the substrate using cryogenic
CO2; and post-treating the substrate using dilute wet chemistry
cleaning.

12. A system for cleaning a substrate having organic and inorganic
residues disposed thereon, comprising: a substrate conveyor; an
atmospheric oxygen plasma jet apparatus including concentric, inner and
outer electrodes through which a mixture of helium and other gases flow
in the presence of a voltage field; and a cryogenic CO2 apparatus.

Description:

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority to U.S. Provisional Patent
Application Ser. No. 61/407,852, filed on Oct. 28, 2010, the disclosure
of which is incorporated herein by reference in its entirety.

FIELD OF THE INVENTION

[0002] The present invention relates to substrate cleaning processes. More
particularly, the present invention relates to an integrated system and
method for cleaning a substrate.

BACKGROUND OF THE INVENTION

[0003] The field of particle, residue removal and surface cleaning in
general extends far beyond the semiconductor industry. Many applications
in biological, medical (implants and equipment), aerospace, imaging,
automotive, pharmaceutical, etc. extensively use surface cleaning as a
preparation step for post or preprocessing. The need for scrupulously
clean wafers in the fabrication of microelectronic devices has been well
recognized since the dawn of solid-state device technology. As
semiconductor device geometry continues to shrink, and wafer sizes
increase, the limitations of existing cleaning methods on devices yield
will become more critical as the size of "killer" particles also shrinks.
In nanoscale manufacturing that need is increased by more than one order
of magnitude. A benign, substrate independent, cleaning process is highly
desirable since it does not have to be modified for different substrates
(as in a chemical based cleaning process) and it does not have a
potential for modifying the surface (such as etching, roughening, etc.).

[0004] Traditionally there are several particle and residual removal
techniques used in semiconductor fabrication and other industries
affected by surface contamination. They include ultrasonic, megasonics,
brush scrubbing, dry Argon ice cleaning, plasma etching, or wet etching,
etc. Effective dry cleaning techniques have been sought after methods in
the industry for the past decade. One dry cleaning techniques that is
film independent is aerosol jets cleaning has been shown to have a good
potential for dry removal of submicron particles. Particles in the gas
stream can be formed by the solidification of liquid droplets or the
gaseous medium during rapid cooling. When the solid particle collides
with the particle, the collision energy may overcome the adhesion force
and remove the particle or residue from the surface. The CO2 aerosol
cleaning technique has been utilized for a wide variety of surface
cleaning applications such as Si wafer, photomask, MEMS devices,
packaging fabrication, imaging devices, metal lift-off, ion implanted
photoresist stripping, disk drives, flat panel displays, and post-dicing
for 3-D stacked IC integration flows.

[0005] In the mask industry, the top critical issues are the cost and
cycle time of mask technology and mask supply. There are numerous yield
loss mechanisms for mask technology: excessive quantity of lithography
defects, un-repairable defects, particle defects, and particle defects
after pellicle mounting. The pellicle is mounted on lithography
photomasks using an adhesive to protect the active area of the mask from
any defects. These masks are utilized to repeatedly print fine features
on masks for high volume products. Mask lifetime is reduced due to issues
like growth of organic layer of defects (also called haze),
electro-static discharge (ESD), non-removable particles, transmission
loss, reflectivity loss, phase change, change in printed critical
dimensions (CD) uniformity, etc. Conventional solvent cleaning techniques
result in degradation of the mask, and hence reduce the mask lifetime.
There is a very tight specification on the mask properties that need to
be maintained for its usage. Foreign material and stains are known as
soft defects on masks that require cleaning. Defects that are found on
masks are not what matters but their printability. Defects flagged by the
inspection tool may not print, or observed defects may not be
electrically pertinent to an active circuit. The concern is some defects
may print due to the particular illumination or focus condition, but may
not be observed during the mask inspection.

[0006] There is a necessity for mask incoming inspection and
re-qualification, due to the repeated printing of defects due to
processing defects of the original mask or degrading defects on the mask
during fab usage (i.e. haze, ESD, and moving particles, etc.). Thus, due
to a multitude of issues, at times, the pellicle needs to be removed from
the mask to implement repairs and cleaning to eliminate the defects that
have resulted in wafer printing errors. Once the pellicle has been
removed, some pellicle adhesive residue is generally remnant. This
residue needs to be completely removed before a new pellicle can be put
in place, once the printing area defects have been eliminated. There are
several pellicle-related issues that also results in mask maintenance
service required like: damaged pellicle, particles under the pellicle,
lithography light exposure-induced degradation, non-removable particles,
etc. UV and EUV exposure-induced degradation of the pellicle glue after a
large number of exposures results in a more stubborn residue after the
pellicle is removed. The ultimate goal is to have a cleaning technique
that will, in a damage-free manner, remove all pellicle glue residue as
well as all soft defects that could be both organic as well as in-organic
particles.

[0007] Known cleaning methods are typically based on wet cleaning that
could result in chemical attack to structures (or in some cases the
utilized chemicals lead to additional problems such as deposit of sulfate
residues of the sulfuric acid, which is well known as one source for
Haze) or dry cleaning mostly with cryogenic CO2, based on the
physical method of momentum transfer, most suitable for inorganic loosely
bonded to substrate, or separately dry cleaning with low-pressure plasma
dry clean that involves active gas-solid chemistry to remove organic
residues (which conventionally performed in reduced atmosphere sometimes
called "ashing").

[0008] There are clear advantages to integrating dry cleaning methods that
provide chemistry to remove organic as well as inorganic particles or
residues effectively. This would be particularly advantageous if done at
near atmospheric pressures, thereby avoiding complicated and expensive
vacuum technology. Combining methods will thus enable removal of all
possible defects in one unit, fast and economically attractive. One
important example would be an application common to the photomask
industry.

[0010] Embodiments of the present invention advantageously provide systems
and methods for cleaning a substrate having organic and inorganic
residues disposed thereon. In one embodiment, the method includes
removing organic residue from the substrate using atmospheric oxygen
plasma, and removing inorganic residue from the substrate using cryogenic
CO2. In another embodiment, the system includes a substrate
conveyor, an atmospheric oxygen plasma jet apparatus including
concentric, inner and outer electrodes through which a mixture of helium
and other gases flow in the presence of a voltage field, and a cryogenic
CO2 apparatus.

[0011] There has thus been outlined, rather broadly, certain embodiments
of the invention in order that the detailed description thereof herein
may be better understood, and in order that the present contribution to
the art may be better appreciated. There are, of course, additional
embodiments of the invention that will be described below and which will
form the subject matter of the claims appended hereto.

[0012] In this respect, before explaining at least one embodiment of the
invention in detail, it is to be understood that the invention is not
limited in its application to the details of construction and to the
arrangements of the components set forth in the following description or
illustrated in the drawings. The invention is capable of embodiments in
addition to those described and of being practiced and carried out in
various ways. Also, it is to be understood that the phraseology and
terminology employed herein, as well as the abstract, are for the purpose
of description and should not be regarded as limiting.

[0013] As such, those skilled in the art will appreciate that the
conception upon which this disclosure is based may readily be utilized as
a basis for the designing of other structures, methods and systems for
carrying out the several purposes of the present invention. It is
important, therefore, that the claims be regarded as including such
equivalent constructions insofar as they do not depart from the spirit
and scope of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014] FIG. 1 is a schematic of the atmospheric-pressure plasma jet
apparatus, according to an embodiment of the present invention.

[0016]FIG. 3 is an optical image of adhesive film after exposure to
oxygen plasma.

[0017] FIG. 4 depicts residues of adhesive left after exposure to oxygen
plasma.

[0018] FIG. 5 presents a combination of local atmospheric plasma and
CO2 clean sources, according to an embodiment of the present
invention.

DETAILED DESCRIPTION

[0019] The invention will now be described with reference to the drawing
figures, in which like reference numerals refer to like parts throughout.

[0020] Embodiments of the present invention advantageously remove
localized organic residue, such as, for example, glue, etc., by
atmospheric oxygen plasma jet apparatus (oxygen plasma) without using
reduced pressure that requires expensive vacuum equipment. In some
embodiments, a coolant (e.g. Liquid N2) shower or spray
pre-treatment, for cooling the residue, may be applied before cleaning.

[0021] The present invention provides various combinations of cleaning
methods, including combining atmospheric oxygen plasma removal with
CO2 cleaning for complete removal of residues, combining submerging
the substrate to be cleaned in benign cooling agents, such as liquid
N2 as a pre-treatment, with atmospheric plasma cleaning for complete
removal of residues, combining submerging a substrate in benign cooling
agents, such as liquid N2 pre-treatment, with CO2 cleaning for
complete removal of residues, combining submerging a substrate in benign
cooling agents, such as liquid N2 pre-treatment, with atmospheric
plasma cleaning followed by CO2 cleaning for complete removal of
residues, combining atmospheric oxygen plasma removal with wet solution
chemistry cleaning, in order to reduce the exposure (or process time)
and/or milder etchant to minimize damage to active structures, combining
submerging a substrate in benign cooling agents, such as liquid N2
pre-treatment, with CO2 cleaning and followed with dilute chemistry
cleaning for complete removal of residues, combining submerging a
substrate in benign cooling agents, such as liquid N2 pre-treatment,
with dilute chemistry cleaning for complete removal of residues,
combining submerging a substrate in benign cooling agents, such as liquid
N2 pre-treatment, with CO2 cleaning and with dilute chemistry
cleaning for complete removal of residues, combining submerging a
substrate in benign cooling agents, such as liquid N2 pre-treatment,
with atmospheric plasma followed with dilute chemistry cleaning for
complete removal of residues, combining atmospheric plasma cleaning with
a CO2 cleaning and followed with dilute chemistry cleaning for
complete removal of residues, combining CO2 cleaning with organic
solvent addition, mask heating, LN2 spray or atmospheric plasma
cleaning; other combinations and permutations of these cleaning methods
are also contemplated by present invention.

[0022] These inventive cleaning combinations provide many advantages over
known substrate cleaning methods. For example, no degradation of the
substrate, such as a mask, is expected during an integrated plasma plus
CO2 cleaning for removal of organic residues, such as pellicle glue
or other contaminates. For CO2 only cleaning, stubborn residue
generally requires copious amounts of CO2 as well as a very long
process time (>1 hr). By pre-application of local atmospheric plasma,
the CO2 consumption can be minimized and the process time can be
drastically reduced, which advantageously reduces cost of ownership
(CoO).

[0023] Using these inventive integrated cleaning methods, stubborn residue
that would require a very aggressive wet clean recipe (using up most of
the degradation budget that is available for the mask) is either
completely preserved or minimized. A reduced chemistry (mild) wet clean
may be used in conjunction if there is any residue after these integrated
techniques.

[0024] In one embodiment, removal of adhesive residue using "dry" cleaning
methods can be automated, which has obvious advantages compared to "wet"
chemistry that can un-intentionally attack substrate areas that are
sensitive to aggressive cleaning agents.

[0025] Initial cleaning with oxygen plasma includes exposing the glue area
to a local atmospheric plasma jet. The jet apparatus 10 includes two
concentric electrodes, inner electrode 12 and outer electrode 14, through
which a mixture of helium and other gases flow. Applying 13.56 MHz RF
power to the inner electrode 12 at a voltage between 100-250 V, ignites a
gas discharge and plasma is generated.

[0026] The ionized gas from the plasma jet exits through nozzle 16, where
it is directed onto a substrate a few millimeters downstream. Under
typical operating conditions, the gas velocity is about 10 m/s with the
effluent temperature near 150 C. While one known process measures the
ozone concentration in the effluent of the plasma jet at different
distances from the nozzle and found that it varied from
2-5×1015 cm-3, the present invention develops an O atom
concentration that equals 8×1015 cm-3 at the nozzle exit,
which gradually falls two orders of magnitude over a 10-cm distance
downstream. The concentration of metastable oxygen is about
2×1013 cm-3 at the exit of the nozzle, which increases to
a maximum at 25 mm, and slowly drops off. The O atoms, and possibly the
metastable O2, may be the active species in polyimide etching.
Assuming atomic oxygen concentration ˜1015 cm-3, and flow
velocity of 10 m/s as estimated above, flux of atomic oxygen on the
sample could reach as high as 1×1018 atoms/cm2-s. If the
reaction probability is assumed to be as low as 1%, the rate of glue
residue removal will be at least 1×1016/1014=102
layers/s or ˜2 μm/minute.

[0027] The rate of oxygen removal of acyclic adhesive by locally
depositing on an Si wafer covered with 3000 A SiO2 film has been
determined. The film thickness was estimated by atomic force microscopy
(AFM) to be at least 2.6 μm. FIG. 2 shows the cross sectional the film
variation.

[0028]FIG. 3 presents an optical image of the acrylic adhesive film
exposed to atmospheric pressure plasma for 40 sec. Visually, the inner
oval area that was exposed to oxygen plasma shows effective adhesive
removal.

[0029] Detailed examination of the exposed area with AFM reveals the
existence of patches of glue resides with heights of up to few nanometers
(FIG. 4).

[0030] In the mask industry, residues of these magnitudes may be tolerated
as may not interfere with re-gluing the pellicle on the same area.
However, these residues can be easily removed with either a rapid
exposure to conventional wet chemistry or preferably by dry physical
techniques, such as CO2 aerosol methods.

[0031] One embodiment of a combined plasma/CO2 cleaning method is
shown schematically in FIG. 5. The substrate 20 moves to left while the
plasma cleaning source 22 and the CO2 cleaning source 24 remain
stationary. Alternatively, the substrate 20 may remain stationary while
the cleaning sources 22, 24 are moved to right. The plasma cleaning
source 22 removes or loosens organic residue 30, followed by the beam
from the CO2 cleaning source 24, which removes loosened organic
residue 30 and/or inorganic residue 32.

[0032] The many features and advantages of the invention are apparent from
the detailed specification, and, thus, it is intended by the appended
claims to cover all such features and advantages of the invention which
fall within the true spirit and scope of the invention. Further, since
numerous modifications and variations will readily occur to those skilled
in the art, it is not desired to limit the invention to the exact
construction and operation illustrated and described, and, accordingly,
all suitable modifications and equivalents may be resorted to that fall
within the scope of the invention.